The fourth version of Apple’s newly renamed mobile operating system arrived Monday, as iOS 4 became available for download.

Apple says the update delivers more than 100 features aimed at end users, as well as 1500 APIs for mobile app makers. Leading the parade of changes in iOS 4 is multitasking support, which will allow users to quickly switch between apps and let apps have limited background functionliaty. Other changes include folders for organizing apps, new Mail features, enhancements to the Photos app, and the ability to change the background wallpaper on your home screen.

iOS 4 is available for download by iPhone 3GS and 3G users, as well as by second- and third-generation iPod touch users. The new OS will also be included with the iPhone 4, which hits retail shelves on Thursday. Some features of the OS won’t be available on older devices—the iPhone 3G and second-generation touch don’t support multitasking, for example. The original iPhone, first released three years ago, won’t support iOS 4 at all. As for iPad users, they won’t be able to upgrade to the new OS until later this fall; Apple hasn’t set a firm date for iPad compatibility.

The iOS 4 update can be downloaded from iTunes. The size of the update will vary depending on what kind of device you use—one Macworld contributor reported that the update for his iPhone 3GS was 378MB, while another editor’s iPhone 3G update was 292MB. The update requires iTunes 9.2, which Apple released last week.

The upgrade process seemed to be going smoothly Monday morning, but iPhone users will be watching closely to see if that continues throughout the day, with last week’s iPhone 4 pre-order problems fresh on their minds. Last Tuesday, a flood of customers placing orders for the new iPhone caused AT&T’s online preorder system to collapse. While reporting that it had taken 600,000 orders for the iPhone 4, Apple also acknowledged that many customers were turned away or “abandoned the process in frustration;” the company has apologized to users affected by the problems.

Past major mobile OS releases have had their own share of difficulties. In 2008, Apple released the iPhone 2 update and iPhone 3G on the same day; the crush of customers simultaneously downloading the new OS and trying to activate phones led to delays and frustration. Last year, the company released the iPhone 3.0 update a few days before the iPhone 3GS’s release, eliminating many of the delays from the year before. Apple is obviously looking to repeat that relatively smooth launch in 2010, by rolling out iOS 4 two days before the latest iPhone arrives.

Apple first took the wraps off the operating system update—then called iPhone OS 4.0—at an April press briefing. That event first outlined the major changes to the mobile OS, including multitasking, folders, and updates to Mail, among other changes. Apple revisited the OS update during Steve Jobs’s keynote at the Worldwide Developers Conference earlier this month; it was there that the company revealed the name change to iOS along with a few more features, most of which are coupled with the upcoming iPhone 4 release.

By far the most eagerly anticipated change in the new OS is the newly added support for multitasking. Other mobile platforms—notably Google’s Android OS—feature multitasking, so its absence from iOS-powered devices has stood out. Apple maintains that multitasking can drain battery life and that it held off implementing the feature in a more battery-friendly way.

The company’s approach to multitasking, introduced with iOS 4, involves a mixture of app-switching features and background processes managed by the operating system. A double-press of the phone’s Home button brings up a Dock with recently used apps; users can switch between them without having to return to the Home screen. The app users leave behind is kept in what Apple calls “a quiescent state in the background”—essentially frozen until users return to where they left off. With iOS 4, Apple also is giving developers several tools that allow them to alter their apps to perform some tasks in the background.

Another addition in iOS 4 is folders, which Apple has added to help users manage the growing number of apps on their mobile devices. Users can create a folder to group together similar app—games, for example, or news and media applications. The addition folders means that iPhone and iPod touch users can have as many as 2000 apps on their phone, as opposed to 180 apps under the previous operating system.

The Mail application gets an overhaul in iOS 4, with a unified inbox leading the changes. That feature will let users see all incoming messages from multiple account. Threaded messages are also supported in iOS 4 for better tracking of replies and follow-ups. Tapping files attached to e-mails in the new OS allows them to be opened in separate applications, including third-party offerings.

Some features in iOS 4 are tailored to the next iPhone, most notably support for FaceTime video chats. Phone calls made between iPhone 4 users will feature a button that lets them talk to face with face over Wi-Fi with the assistance of that phone’s front-facing camera. The new OS will also support Apple’s iMovie for iPhone app, a $5 video-editing app that will only run on the iPhone 4.

Other iOS 4 features of note include support for an iPhone version of the iBooks e-reader app; a built-in spell-checker for Mail, Notes, and other apps; custom playlist creation; Bluetooth keyboard support; and the ability to change the default search engine in the Safari Web browser from Google to Microsoft’s Bing. (Users can already choose between Google and Yahoo.) The Camera app in the iPhone adds support for 5x digital zoom and, for video-capable phones, tap-to-focus video, while the Photos app gains support for the face-recognition and geolocation features familiar to iPhoto users on the desktop.

As with previous updates, iOS 4 will be a free download for owners of those iPhones that support it. However, in a first, it will also be free for iPod touch owners, thanks to changes in accounting rules.

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